Only, you know, sometimes it doesn't.
There are brighter days, but there will probably be some darker days too. Life is full of loss, love, delight, pain, hurt, discovery, boredom, desire, fun. It's not a graph that has a steady incline, it's a jumbled mess of ups and downs that never stays the same way for very long.
So yes, if things are really bad right now, they will get better. And then they'll probably get bad again. And then better again. And right now, in the midst of the bad, know that there's Someone good who will be with you in all of the ups and downs.
(This was inspired by the following piece from Radiolab. Started listening to this podcast when I run, but heard this one in the car and it took my breath. Taken from David Eagleman's brilliant book Sum: Forty Tales from the Afterlives. Seriously, don't click away from this page without listening, because it is gorgeous. The piece starts at about 1:30.)